The present invention relates to a process for laser marking metal surfaces. In particular, the invention relates to a method of laser marking which comprises applying a laser beam to a metal surface under the influence of an assist gas to produce durable, repeatable and striking colours on the metal surface.
Metals such as titanium, stainless steel and magnesium are widely used in many areas, such as in the manufacture of recreational and personal items. Such items may include, for example, camera casings, mobile phones, sporting goods, jewellery, watch cases, eye-glass frames, tie-pins, hair pins, souvenirs and so on. The cosmetic appearance of these items or products is of recognised importance to their commercial success. Furthermore, personalisation of such products is becoming increasingly desirable. Laser marking is regarded as a highly flexible process for creating patterns on articles, including metal articles. However, conventional laser marking techniques engrave on metal surfaces to form rough grooves with brown or black burn marks to create the marking contrast. These marks are not generally attractive from the cosmetic view point.
Printing and emulsion coating are also common techniques used for the decoration of metal surfaces. However, scratch and wear resistance of such coatings and the fading of colours of these coatings with time are recognised problems associated with these coatings. Hard coatings, such as of TiN have also been used for protective and decorative applications. Deposition of such hard coatings is generally achieved by flame and plasma spraying, sputtering and vacuum evaporation or the like. However, these coatings often have coarse surfaces and provide poor uniformity. Furthermore, using such techniques multiple steps are required to create coatings of multiple colours. In this regard, inflexibility in changing the applied colours and patterns generally makes these techniques unsuitable for product personalisation.
Decorative coatings on metal surfaces may also be prepared by electrochemical treatments in aqueous electrolytes. Such techniques generally employ certain voltages and electrical currents as described in U.S. Pat. No. 4,869,789. In these processes, changing the metal ions in the electrolyte provides for changes in colours applied to the metal surface. This process has been used in, for example, the jewellery industry, and is more commonly used for the anodising of titanium to create colour coatings. However, flexibility of changing the applied colours and patterning is limited.
According to one aspect of the present invention, there is provided a process for laser marking a metal surface comprising:
applying a laser of predetermined wavelength and beam energy to said metal surface, said metal surface, during the application of said laser having an assist gas directed thereon at a predetermined gas pressure and flowrate to facilitate controlled oxide film formation on said metal surface where said laser is applied.
The invention also provides metal surfaces including a mark applied by the process described in the immediately preceding paragraph, or a substrate or article including such a metal surface.